-Check Grounds
-Are you near fluorescent lighting?
-Are you near lost of AC outlets?
-Stand away from your amp
-Reduce gain, if needed
-Volume level for the room size

I dont believe in potting personally, I have never run into an issue with modern pickups that has required them to be potted. My Explorer has some of Gibson's hottest passive pickups in it, they are not potted and I have no issues with feedback at all. I even unpotted the bridge pickup in my Les Paul to give it more bite.

After you run through ne14t's list, if you still have feedback issues, then I would ask your local hardware store if they have copper shielding tape. Put that on the walls of your guitar cavity. Make sure that no unshielded (read: bare) wires are touching the copper shielding tape. That kind of tape will reduce outside electrical interference.

I'VE DONE IT and it does help as long as potting is actually the problem! You'll find that most humbuckers in the last maybe 15 years have been potted by default, but some older ones might not have been. In some old MIJ Les Paul's I've had, the pickups haven't been potted, but dunking them in melted wax for 20 minutes solved all the feedback problems

Check your grounding, power, and distance from amp anyway, but if it still does it then pot those pickups. If you only get the feedback at high volume or with distortion, then that's probably your problem